if you are new to giving "IM" (intramuscular) injections...lets play it safe and go for the peducle (stump of the tail so to speak). If you slip, go to far, or even push straight through the tail (actually...this isnt as bad as it sounds, relatively speaking here)...not a biggie.
If this were a fish, any fish, for example, that wasn't eating and I suggested (truly) you give it a vitamin B shot...the does is not a big deal. Water soluble and very hard to overdose. Baytril however IS NOT to be casually administered. A little bit of the right amount can cure a raging infection that would decimate the this otherwise and perhaps other fishes int he system. Slightly more than the right amount, however, will shut down the fish's kidneys and kill it. We gotta weigh the fish...
A few ways to do this here, but let me ask for simplicity if you have a digital kitchen scale? We need a gram scale. This fish may be large enough to register a useful number on a digital kitchen scale.
You need to by some needles too: 23G X 1" would be the size here...bigger needles (smaller numbers such as 19G will be too large of a gauge). If you don't have a source, go back to your local vet, advise him what you are doing here. We can reference aquatic citations to back up the purpose here. They can sell you some needles (disposable...never reuse) and a syringe. Mind you, the Baytril will come in a multi-use bottle (rubber cap you pierce with the needle to draw from...hence not wanting reused needles going into the uncontaminated solution). Vets use Baytril for everything from birds to horses. Very easy to find.
According to famous fish doc Ed Noga, the dose for Baytril (enrofloxin) is 10 mg/kg per injection every third day.
You'll need to do some math depending on the strength of Baytril you find - or ask the kind vet people to do it for you; they do it daily on their own. Tell them the weight of the fish and the above dose to convert for with the strength of Baytril they have on hand.
Heed the once every 3 day dose. More is not better here, again.